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Its availability as an e-book added to its popularity, meaning coy readers could enjoy it without having to be seen buying or reading it.

However, its risque subject matter has not deterred millions from happily brandishing their own copies in cafes, trains and on beaches.

Mills & Boon's Twelve Shades of Surrender, billed as ‘a daring selection of stories for fans of 50 Shades', can be downloaded for £1.99 each.

Zeitgeist: Shirley Conran's Lace is being released after 15 years out of print

The dozen titles include Cuffing Kate, a fantasy about dominance and submission, and For Your Pleasure, about naughty neighbours.

The firm, famed for its swooning romantic stories, began publishing more racy content in 2008, with the Spice paperback series.

But the boom in digital e-readers such as Amazon's Kindle, Sony's Reader, and tablets such as the iPad, has given the sexy novel market a lucrative new lease of life.

A Mills & Boon spokesman said: ‘Erotica is becoming more mainstream and acceptable as we and others evolve our packaging to become less explicit and women become ever more empowered to read what they want to and enjoy.'

The revival of Shirley Conran's Lace, three decades after it was first published and 15 years after it went out of print, is also a nod to the demand from women for books with sexual content designed for them.

It was a huge hit in the 1980s despite the controversy surrounding its explicit content and was made into a mini-series for TV.

Writer Victoria Fox, who has had success with erotic novel Hollywood Sinners and is about to launch new title Temptation Island, told the Daily Mail that James had made literary sex fashionable again.

She told the Daily Mail: 'Hallelujah! Sex is back - and more important than that, women are loving it. This appetite for fantasy and escapism could be a sign of the recession - I believe, in part, it is - but more likely is the truth that women are unafraid of their sexuality and want to experience romance in a new way.'